The present proposal will investigate the role of serotonin in modulating territorial aggression in a free-living animal. Serotonin levels are inversely correlated with aggressive behavior, and they have been experimentally shown to regulate aggression. This study will expand on a large base of knowledge about the behavior and hormonal control of aggression in the song sparrow Melospiza melodia morphna. The males of this species display year-round territorial aggression in both a breeding and nonbreeding context. While superficially similar, the underlying hormonal mechanisms of the behavior are different. Gonadal steroids are more important for aggression in the breeding context while nongonadal steroids regulate nonbreeding aggression, thus, this species acts as a natural "knockout" allowing a direct comparison of serotonergic modulation under two different types of steroidal regulation. Song sparrows can be experimentally manipulated in the field, thus setting up a unique and powerful model to study the neuroendocrine control of territorial aggression in situ and in multiple ecological and hormonal contexts. This research will provide valuable new insight into the steroidal regulation of the serotonergic system. In addition, this study will advance our understanding of how the natural environment influences aggressive behavior.